The all-iron flow battery initially proposed in 1981 by Hruska and Savinell represents a cost effective and environmentally benign system for large scale energy storage¹. The all-iron chemistry consists of the Fe²⁺/Fe³⁺ redox reaction at the positive electrode, and iron plating and stripping at the negative electrode. Because the standard reduction potential for iron plating is more cathodic than that of hydrogen evolution, electrolyte pH heavily influences coulombic plating efficiency as well as deposit morphology and microstructure^2-5. Increasing the electrolyte pH decreases the diffusion limited current for hydrogen evolution and shifts the equilibrium potential more negative⁶. However, iron is not stable at high pH as shown by the Pourbaix diagram, where it will precipitate as hydroxide species. Boric acid is a common component in iron group plating baths where it is believed to function as a buffering agent helping to stabilize pH near the electrode^7-9. However, it is not understood how boric acid with a pK_a value of 9.27 can buffer at low pH¹⁰. The aim of this work is to further understand the effects of pH near the electrode surface and the role of boric acid on the electrodeposition of iron group metals.

Electrodeposition of selected iron group metals will be discussed with a focus on the effect of boric acid. We present an in-situ technique for monitoring near surface pH as a function of plating current density. This measurement is accomplished using a tungsten microelectrode which acts as an indicator for hydrogen ion activity^11-13. Our results demonstrate that boric acid is not effective in buffering the near surface pH in the acidic baths tested. Even at high current densities in a pH 2.5 electrolyte, the near surface pH did not exceed 6. However, the addition of boric acid improved deposit morphology and current distribution; coupons plated from electrolytes containing boric acid, and those without, are compared and discussed. At high current density and high pH, boric acid incorporates into the deposits, which is studied by use of inductively coupled plasma optical emission spectrometry (ICPOES). Electrochemical impedance spectroscopy, cyclic voltammetry, and coulombic efficiency data aid in understanding additive adsorption as well as the influence of boric acid on the deposition mechanism. In addition to surface specific information, the effects of boric acid on exhaustive plating and long-term bath stability will be presented to account for buffering effects in the bulk electrolyte when pH exceeds 6.

References

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